He said this stretched "across political, legal, banking and accountancy sectors".

The tapes were made between 2005 and 2008 when Mr Cushnahan worked as a consultant for the Graham family's businesses in Belfast.

Although the conversations pre-date the Project Eagle sale, which took place in April 2014, Mr Graham believes their contents are relevant to inquiries being conducted by the PSNI and the UK's National Crime Agency.

Both agencies are investigating claims politicians were due to receive kickback payments in connection with the sale of the loan portfolio.

Mr Graham has also contacted the US Securities and Exchange Commission (SEC), highlighting "inappropriate political relationships and potentially unlawful banking relationships" he alleges are disclosed in the conversations.

He told a hearing of the Northern Assembly's committee on finance and personnel there were potential breaches of the US Foreign Corrupt Practices Act which would merit an SEC investigation.

The allegations were among a series of explosive claims made by the businessman during his evidence to the committee.

Mr Graham also alleged Mr Cushnahan failed to disclose to Nama that he had a 5pc interest in companies whose loans were taken over by the so-called bad bank.

This has previously been denied by Mr Cushnahan, who claimed documents indicating he had the shareholding were "an administrative error".

In his evidence to the committee, Mr Graham also claimed his family's business had been the victim of a "criminal conspiracy".

Former banker Cushnahan, a high-profile business figure in the North for decades, served on Nama's Northern Advisory Committee (NIAC) between May 2010 and October 2013.

His appointment was made by the late former Finance Minister Brian Lenihan following a recommendation from the then Stormont Finance Minister Sammy Wilson.

Mr Cushnahan has been mired in controversy since July when Nama revealed how his involvement in the Project Eagle sale led to one of the bidders, investment firm Pimco, withdrawing from the process.

This occurred after Pimco informed Nama in March 2014 - five months after Mr Cushnahan stepped down from the NIAC - that he stood to be paid Stg£5m in fees if Pimco was the successful bidder. After Pimco withdrew, rival firm Cerberus secured the portfolio.

Mr Graham was one of 55 Nama debtors whose loans were sold to Cerberus as part of the deal. Cerberus subsequently moved to appoint administrators to two of his businesses, a move Mr Graham is currently fighting in Belfast's High Court.

Mr Graham told the committee that Mr Cushnahan was taken on as an adviser in 2005 "on how best to maximise the potential of our family businesses".

As part of the arrangement, Mr Cushnahan became chairman of the SP Graham Group, received a 5pc shareholding, and operated from one if its offices. However, there was "a breakdown in the relationship" in 2008 and Mr Cushnahan departed.

"Instead of developing our business in a manner that we had anticipated, Frank Cushnahan set about trying to sell our business, which would have involved his recouping a significant return on his 5pc," said Mr Graham.

He said that five months after Mr Cushnahan was appointed to the NIAC, loans the group had with Bank of Ireland were moved into Nama.

This occurred despite Mr Graham's belief the debts were not eligible for inclusion in Nama. He said the businesses involved had never missed an interest payment.

"I consider the reason that Bank of Ireland took such a harsh and unfair view of us is because of Frank Cushnahan's malevolent influence," he said.